Thanks to Adrian Humphreys for releasing a few more details about the text messages for us. I was able to clean up the timeline a little bit and fix the ordering of some messages. Here's my current analysis of the conversation between Smich and Millard on May 2 and 3:
On May 2, Millard texts Smich that he will "check him after 4pm" the next day to work on "the mission". Smich then replies, "Ok it's fireworks tomoro night." These two messages clearly indicate that there is a plan to work on "the mission" on May 3, and the plan is known to both of them. Whatever Smich meant by fireworks, the conversation suggests to me that fireworks is somehow related to the mission they are working on.
In the morning of May 3, Smich asks if Marlena and Pedo and can reach and chill while they do their thing, and he sends the photos of the log bench and stools. I make a couple of inferences from this: 1) Smich thinks they will be travelling somewhere (hence wondering if Marlena and Pedo can tag along), and 2) the log bench and stools relate to Marlena and Pedo hanging back and "chilling" while Millard and Smich go off to do their thing, whatever that is.
In the afternoon of May 3, Smich texts "High Five!" along with the photo of the sausages arranged in a hand shape. I agree with others that it's difficult to draw a conclusive sinister inference from this photo, however, in my mind it doesn't completely rule out symbolism of using of the incinerator. From previous messages, it is clear that Smich and Millard speak in vague and symbolic terms when it comes to discussion of the mission. The fireworks, reaching and chilling, and the furniture photos suggest that the plan involved something outdoors-y, IMO.
At 4:18pm on May 3, Smich texts "Wut's good cracka". Up to this point, Millard hasn't responded to him all day, and there seems to be an air of excitement or eagerness on Smich's behalf to work on the mission together.
Millard finally responds at 4:20pm: "just finished a 6 hr meeting at hangar", and Smich immediately replies, "Can u bring me gloves and tape and maybe an orange guy? The first two I need more." Smich's quick reply and the way he jumps straight to business bolsters the idea that he is excited/eager about their plan to work on the mission. The orange guy wasn't so important, but he really needed the gloves and tape.
At 4:56pm (note: I assume AH made a typo since his video states 4:56am) Millard writes, "I'm reaching your ways now, ete 50 mins", suggesting that they did in fact get together that evening.
Between 8:30pm and 10:30pm that night, Millard was at Maple Gate looking up trucks on Kijiji/Autotrader and he also called Araujo and Palmilli. So, based on the earlier text messages, it would seem that Millard picked up Smich and took him to Maple Gate where they checked out trucks for sale.
Based on all the text conversation leading up, I would suggest that their original plan was to try to find a test drive that night (May 3), but it ended up being too late and/or they couldn't book a time with anyone (e.g. Araujo, Palmilli). This would give proper context to the fireworks, gloves/tape, and the excitement to carry out "the mission". Smich thought it was fireworks that night, but it wasn't yet.
Although the plan fell through that evening, they kept pushing for it over the weekend until they eventually got Bosma on May 6.
All my opinion, of course.